1. Field of the Invention
This invention relates to an electromagnetic display or indicator element. The element uses a disc which is contrastingly colored on opposite sides and controlled to selectively display one or the other side in the viewing direction. Thus, the element may be used singly as an indicator or in an array to collectively form a display.
2. Description of the Prior Art
The element with which the invention is concerned is of the type where an disc is provided, rotatably mounted on a housing to rotate about an axis in the plane of the disc, and extending across approximately a central or diametrical line thereof. A permanent magnet is mounted on the disc to define a magnetic axis transverse to the rotary axis and preferably perpendicular thereto. The disc is contrastingly colored on opposite sides. The orientation of the disc, to display one or the other of the contrasting sides in a viewing direction is controlled by a pair of high remanence, magnetically permeable pole piece cores each having a forward end adjacent the locus of the disc permanent magnet. The forward ends of the pole pieces are located so that, when oppositely magnetized they control the disc through the permanent magnet to display one of its sides in the viewing direction. Where the polarity of both oppositely magnetized pole piece cores is reversed, the result is to reverse the orientation of the disc to display its other face in the viewing direction. Control of the magnetization of the pole piece cores is customarily provided by having a yoke joining them rearwardly of the disc with an energizing winding surrounding the yoke to be energized to magnetize the pole piece cores in one or in the other polarity.
One disadvantage of the conventional arrangement is that the collective shape of the cores and yoke makes the provision of an energizing winding on the yoke awkward, difficult and expensive. A further disadvantage is that the high remanence magnetically permeable material of the cores (typically a ferrous alloy) is expensive and the design requires two such cores. A further disadvantage is that, since the yoke runs transverse to the viewing direction, an increase in the winding thickness to provide more turns results in a corresponding increase in the depth of the unit i.e. the dimension measured in the viewing direction. Increase in such dimension creates design and housing problems which are difficult and expensive to solve.